The present invention relates to a process for preparing water-soluble polymers, more particularly to a process for preparing water-soluble polymers comprising partially hydrolyzing N-vinylformamide polymers.
The water-soluble polymers produced by partially modifying N-vinylformamide polymers are known as a cationic high-molecular weight flocculant having excellent flocculating performance, but as regards the manufacturing process thereof, there is known only a method in which the polymers are produced in the form of an aqueous solution. As a method for producing such water-soluble polymer in the form of powder, the present applicant had previously proposed a process comprising subjecting N-vinylformamide to a high-concentration solution polymerization to form a hydrous gel-like polymer, granulating this gel-like polymer, modifying the granulated polymer in methanol in the presence of an acid, and then dehydrating the product by adding methanol, thereby obtaining the objective polymer in a granular form.
This process, however, has the following problems. It is subject to certain limitation in reducing the particle size of the hydrous gel-like polymer, and the acid used for modifying the polymer is needed in an excess amount. Also, the methanol dehydration rate of the modified polymer is low, and further, since the drying rate of the dehydrated polymer by vacuum drying or hot air drying is low, the polymer would often be degraded due to heat load at drying, and worsened in water solubility.
As a result of the present inventors' studies for solving the problems, it has been found that a process for producing a water-soluble polymer comprises granulating the hydrous gel-like polymer obtained by polymerizing an aqueous N-vinylformamide in the form of an aqueous solution of which a monomer concentration is 20 to 60% by weight, dispersing the produced particles in methanol and contacting them with an aqueous solution of ammonium salt of a monovalent acid in an amount of 1 to 200% by mole based on the formamide groups in the polymer to partially modify the formamide groups in the polymer, thereby obtaining a powdery water-soluble polymer having excellent drying characteristics and water solubility, and the present invention has been attained on the basis of this finding.